throw : Java Glossary

To trigger an exception. What happens to handle the exception depends on enclosing
try-catch blocks or failing
that in the enclosing try-catch blocks in the calling method, or its caller etc.

For unchecked Exceptions ( i.e. Errors and RuntimeExceptions such as
IllegalArgumentException) the throwsIllegalArgumentException is
optional. I find them useful documentation since they show up in the Javadoc.